The present invention relates to fuel ejectors and fuel ejection methods for burners used in process heaters, boilers, and other fired heating or incineration systems. More particularly, but not by way of limitation, the present invention relates to fuel ejectors and fuel ejection methods effective for reducing NOx emissions.
A need presently exists for more efficient and economical burner systems capable of significantly reducing NOx emissions from heaters, furnaces, boilers, and other fired heating or incineration systems. One approach employed heretofore has been to precondition the burner fuel by mixing substantially inert flue gases therewith. As used herein, the phrase xe2x80x9cflue gasxe2x80x9d refers to the gaseous combustion products produced by the fired heating system. Diluting the fuel with flue gas reduces NOx emissions primarily by lowering burner flame temperatures.
As will be understood by those skilled in the art, some prior burner systems employ xe2x80x9cfree jetxe2x80x9d fuel ejectors for entraining 5 flue gas in and mixing the flue gas with at least a portion of the burner fuel. The phrase xe2x80x9cfree jetxe2x80x9d refers to a jet flow of a first fluid (i.e., fuel) issuing from a nozzle into a second fluid (i.e., flue gas) which, compared to the jet flow, is more at rest. Free jet fuel ejectors are sometimes positioned for discharging at least a portion of the burner fuel such that, prior to combustion, the fuel stream must travel through the flue gas environment existing within the interior of the fired heating system.
A free jet ejector 10 of a type heretofore employed in some burner systems is depicted in FIGS. 1 and 2. Ejector 10 comprises: a fuel pipe 12 which extends into the interior 20 of the heating system through a furnace wall or other structure 11; an ejector tip or nozzle 15 secured on the distal end of fuel pipe 12; a flow cavity 17 within ejector tip 15 in fluid communication with the flow passageway of fuel pipe 12; and an ejector port 14 extending laterally from flow cavity 17 through the sidewall of ejector tip 15. The lateral cross section of burner tip 15 will typically have a round shape, as depicted in FIG. 2.
Ejector port 14 discharges a stream of fuel 16 toward a combustion zone (not shown) within the fired heating system. The fuel will typically be a fuel gas comprising natural gas or generally any other type of gas fuel or gas fuel blend employed in process heaters, furnaces, boilers, or other fired heating or incineration systems. The fuel stream 16 flows through and entrains flue gas present within the interior 20 of the fired heating system.
It is typically desired that as much flue gas as possible be entrained in and mixed with fuel stream 16 as it travels toward the combustion zone. However, such entrainment and conditioning must typically occur very quickly and over a relatively short distance.
Unfortunately, the fuel ejectors heretofore used in the art have not provided optimum or adequate flue gas entrainment in the fuel discharge region 18 immediately outside of the ejector port 14. Because of the shape of ejector tip 15, the furnace flue gas flowing into the ejector discharge region 18 must contact and interact with fuel stream 16 at a very abrupt angle (typically close to 90xc2x0). In addition, the flue gas 19 flowing into discharge region 18 around the exterior of ejector tip 15 must follow a very sharply curved flow path. As a result of these characteristics, eddies and currents are created in discharge region 18 which significantly reduce flue gas entrainment.
As will thus be apparent, a need exists for a fuel ejector which will provide significantly enhanced flue gas entrainment, particularly in the discharge region 18 of the fuel flow stream.
The present invention satisfies the needs and alleviates the problems discussed hereinabove. The present invention provides an improvement for an ejector having at least one port effective for delivering a flow of fuel into a heating system such that flue gas within the heating system is entrained in the flow of fuel. In one aspect, the inventive improvement comprises the ejector having an aerodynamic shape effective for increasing entrainment of the flue gas in the flow of fuel in the discharge region at the ejector port.
In another aspect, the inventive improvement comprises the cross-sectional shape of the ejector in a cross-sectional plane extending through the ejector port having: a discharge end wherein the port is provided; a major axis extending through the discharge end; a second end on the major axis opposite the discharge end; a total length along the major axis from the discharge end to the second end; and a maximum lateral width which is less than the total length. In addition, the lateral width of the cross-sectional shape preferably increases along the-major axis from the discharge end to the location of maximum lateral width.
In another aspect, the present invention provides a method of reducing NOx emissions from a heating system having flue gas therein. The inventive method comprises the step of ejecting a fuel into the heating system in free jet flow from at least one port of an ejector positioned in the heating system. The free jet flow has a region of discharge adjacent the port and the ejector has an aerodynamic shape effective for increasing entrainment of the flue gas in the free jet flow at the region of discharge.
In yet another aspect, the present invention provides a method of reducing NOx emissions from a heating system having a flue gas therein comprising the step of ejecting a fuel into the heating system in free jet flow from at least one port of an ejector positioned in the heating system. The cross-sectional shape of the ejector in a cross-sectional plane extending through the port includes: a discharge end wherein the port is provided; a major axis extending through the discharge end; a second end on the major axis opposite the discharge end; a total length along the major axis from the discharge end to the second end; and a maximum lateral width which is less than the total length. The maximum lateral width is located along the major axis at a location of maximum lateral width and the cross-sectional shape increases in lateral width from the discharge end to the location of maximum lateral width.